It is feared that her presence in Portugal will provoke a media circus and further aggravate Portuguese authorities as the Scotland Yard team seek cooperation to carry out their own inquiries on Portuguese soil.

A source said: "Because of the sensitivity of the situation, police don't want the McCanns anywhere near the country because they fear it could turn into a media circus.

"The Met police are worried it will create a bigger platform for Mr Amaral and his supporters to spout more bile and could damage their own inquiries in the country.

"It will be bad enough having all the old allegations dragged up again and repeated. No doubt Mr Amaral will want to say his bit outside the court and it will all flare up again. It will not be helpful for the Yard."

Madeleine disappeared from an apartment in Praia da Luz shortly before her fourth birthday in May, 2007

Neither Mr nor Mrs McCann from Rothley, Leicestershire, are due to give evidence in the libel trial against Mr Amaral, who headed the initial investigation into their daughter's disappearance which led to the couple being named as suspects.

Instead, Dave Edgar, a private investigator hired by the family to search for Madeleine and Michael Wright, husband of Mrs McCann's cousin Anne-Marie, will tell the hearing how Mr Amaral's "poisonous lies" have nearly destroyed the family.

Madeleine, who disappeared from an apartment in Praia da Luz shortly before her fourth birthday in May, 2007.

The McCanns, who are both 45, were finally cleared in July 2008, when the Portuguese police investigation was shelved for lack of evidence.

But the following month, Mr Amaral who was sacked from the investigation and has since left the police force, published a book accusing them of faking their daughter's abduction to cover up her death in the apartment.

In 2010, they won a court battle in Lisbon to ban sales of the book entitled "The truth of the Lie", a ruling that was overturned later that year.

Mr McCann, a heart specialist and Mrs McCann, a former GP, are now seeking damages for themselves and their twins, Sean and Amelie plus further damages for the harm caused to the search for Madeleine, who would now be ten years old.

In a 36-page writ, first lodged in June 2009, they describe Mr Amaral as a self obsessed, manipulative money-grabber and accuse him of libel and breaching their human rights.

In July Scotland Yard announced a full-scale investigation into the girl's disappearance after identifying 38 potential new suspects and said they had requested assistance from Portugal's Policia Judiciaria.

A team of 37 detectives has spent the last two years examining thousands of documents, witness statements and evidence reports as part of a government funded review of the case ordered by Prime Minister David Cameron.